A positive temperature coefficient (PTC) material exhibits a positive temperature coefficient effect that renders the same to be useful as a protecting device, such as a fuse.
Referring to FIG. 1, a conventional over-current protection device includes a PTC polymer material layer 81 and first and second electrodes 82, 83 formed on two opposite surfaces of the PTC polymer material layer 81. The PTC polymer material layer 81 includes a polymer matrix of a polymer material that contains a crystalline region (not shown) and a non-crystalline region (not shown), and a conductive particulate filler (not shown) that is dispersed in the non-crystalline region of the polymer matrix and that is formed into a continuous conductive path for electrical conduction between the first and second electrodes 82, 83. The PTC effect is a phenomenon that when the temperature of the polymer matrix is raised to its melting point, crystals in the crystalline region start melting, which results in generation of a new non-crystalline region. As the new non-crystalline region is increased to an extent to merge into the original non-crystalline region, the conductive path of the conductive particulate filler will become discontinuous and the resistance of the PTC polymer material will be sharply increased, thereby resulting in an electrical disconnection between the first and second electrodes 82, 83.
The operating temperature of the over-current protection device may be adjusted by adjusting the composition of the polymer matrix. However, the operating temperature can only reach a value within a range of from bout −40° C. to about 85° C. when the polymer matrix is made from polyolefin.
In order to achieve a higher and broader range of operating temperature, such as between −40° C. to 125° C., the polymer matrix may be made from polyvinylidene fluoride (PVDF).
U.S. Pat. No. 5,451,919 discloses a conductive polymer composition having a resistivity at 20° C. of less than 10 ohm-cm. The conductive polymer composition contains at least 50 vol % of polyvinylidene fluoride and 1 to 20 vol % of a second crystalline fluorinated polymer. The conductive polymer composition was compression-molded to form a plaque. The plaque was laminated on two sides with electrodeposited nickel foil to form a laminate. The laminate is irradiated to a dosage of 10 Mrads using an electron beam. The polyvinylidene fluoride is under a trade name of Kynar. However, the operating voltage or the breakdown voltage of the over-current protection device including the aforesaid conductive polymers can only reach 30 Vdc according to thermal runaway test.
In view of the foregoing, there is a need to improve the operating temperature as well as the operating voltage of an over-current protection device.